Faithful transmission of the genome through gametes is critical for fertility, health of offspring, and success of the species. Errors in meiosis can lad to aneuploidy, chromosome aberrations, or gene mutations. The vast majority of spontaneous abortions due to aneuploidy are traceable to Meiosis I defects that occur during oogenesis. Although checkpoint mechanisms exist to stimulate repair of meiotic errors and eliminate irreparably defective meiocytes, their molecular identities and mechanisms are poorly characterized or unknown in humans or mammals. We have used genetic analyses of mice to identify the key checkpoint responsible for responding to DNA double strand breaks (DSBs) in oocyte meiosis. DSBs are enzymatically induced at the beginning of meiosis in order to stimulate meiotic recombination - a process essential for proper chromosome segregation upon cell division. Failure to repair the DSBs triggers a checkpoint that results in oocyte elimination. We identified checkpoint kinase 2 (CHK2; CHEK2) as an essential component of this response. Startlingly, female mice that are oocyte-deficient from a mutation that disrupts DSB repair can have their fertility restored by concurrent mutation of Chk2, yielding apparently normal offspring. We also found that both programmed and induced DSBs trigger CHK2-dependent phosphorylation of p63 (TRP63) in diplotene oocytes. These and other data establish CHK2 as essential for DNA damage surveillance in female meiosis, and we propose that the DNA damage checkpoint pathway is: ATR>CHK2>p63. However, Chk2 is not essential for elimination of DSB-bearing spermatocytes, indicating sexual dimorphism. This proposal builds on these findings with the following basic and translational objectives: 1) To validate and mechanistically characterize putative upstream (ATR) and downstream (p63) components of the oocyte DNA damage checkpoint using conditionally mutant mice and molecular analyses; 2) Determine the identity of the male DNA damage checkpoint; 3) Determine the fate of DSBs in recombination-defective, rescued oocytes by whole genome sequencing of offspring and genetic analyses of potential alternative DNA repair pathways. This is important for assessing the safety of checkpoint inhibition for reasons of assisted reproduction or fertility preservation in cancer patients; and 4) Determine if CHK2 drug inhibitors can protect oocytes from death due to cancer treatments, and in a manner that doesn't cause birth defect-causing mutations. In sum, this project will elucidate the genetic quality control mechanisms operate in mammalian gametes. Arguably, this is one of the most important processes for the health and success of our species and others.